Can the rear input of the MP-1 be modded to accept Line Level signals?
Probably, the early Mp1s had the back panel inputs as line level, but MJMP will know better than me here...
Personally, I'd put the MPX-G2 in the Mp1 loop (not stereo I know, If you want to keep stereo effects (and I would)) then with a MP1 you could use the ART splitter (see other posts search for ART, I've babbled on at great length about this) BTW IMHO Lexicon make great reverb gadgets!!!!. Putting a MP1 in another devices effects loop (and yes it works) is INMO a really bad idea (MP1 great device, other device not so good, can't be as good... ever....) unless you are using a MP2 loop just to get the MP2 cab sims on the MP1 output, (I tested this last week, works very well, sounds great).
I think (from my experience on the board so far) what people seem to have trouble working out, is that the ADA preamps are (f#ing) awesome (well, maybe they don't ;-) ), and the majority of other stuff isn't that good (lots of crap), obviously some slightly better than the other. There's a paradigm shift here also, tone cloners (I'm not convinced!!!) and real tube preamps... the AxeFX is the only new preamp that comes anywhere near to something I'd accept, (that I know of, and it doesn't cut it live).
Long story short; use the lexicon as an effect (that's it's job and it's very good at it (just use delay, reverb, etc)), use you MP1 as your guitar preamp, (it's good/great at that). I love lexicon gear, but they will never make (a big statement I know) a tube guitar preamp to better the ADA MP1/2...